 2009-2011 Microchip Technology Inc. DS01260B-page 1

AN1260

INTRODUCTION

The rechargeable Li-Ion/Li-Poly batteries are widely

used in today’s portable Consumer Electronics (CE).

Commonly seen Li-Poly batteries are Li-Ion Polymer

batteries that use a solid polymer separator and share

the same charge algorithm with Li-Ion batteries. Thus,

Li-Ion batteries will represent both Li-Ion and Li-Poly

batteries in this application note. Because of the

growing features and the increasing size of the display

in a portable electronic product, the battery usage is

also modifying. The daily battery charging frequency

increases, and it becomes important to operate a

device while charging its battery.

The traditional method to design a battery-powered

system is to connect the system load directly on the

battery. The system load continuously discharges the

Li-Ion battery and costs a battery’s life cycle. In order to

maximize the life cycle of Li-Ion batteries, it is

recommended to terminate the charge properly and

power the system from the input power supply, when it

is available. To prevent overcharging Li-Ion batteries,

an elapse timer is usually required as a secondary

method to turn off the battery charge activities, before

a proper termination condition is met. Minimum current

detection during the Constant Voltage (CV) stage is the

typical termination method for Li-Ion batteries. If a

system is constantly drawing current out of a Li-Ion

battery, the charge management system will never be

terminated properly by minimum current. It can turn on

and off periodically, or result in an error by a timer-fault

condition.

Microchip’s MCP73871 was developed to overcome

these design challenges of Li-Ion battery-powered

applications. The MCP73871 is a monolithic solution

that offers compact size and rich features. It is an ideal

candidate to design in small form-factor systems, while

extending the system runtimes and battery life.

This application note is intended to offer detailed

design guidance for portable electronics designers who

are interested in taking advantage of using Microchip’s

MCP73871 in their projects. The MCP73871

demonstrates strategies to deliver Li-Ion charge

management solutions in a short time, satisfying space

and cost concerns.

FIGURE 1: Typical MCP 738 71 App licat ion .

Author: Brian Chu

Microchip Technology Inc.

System

Load

Low

High

Low

High

Low

High

Low

High

VSS

STAT2

SEL

STAT1

VPCC

PROG2

LBO

Single-Cell

Li-Ion

Battery

OUT

VBAT

THERM

VBAT_SENSE

PROG3

PROG1

1, 20

14, 15, 16

18, 19

10,11(EP)

OUT

NTC

10 μF

4.7 μF

110 kΩ

10 kΩ

RPROG3

RPROG1

470Ω

AC-DC Adapter

or USB Port

330 kΩ

Li-Ion/Li-Poly Battery Charge and System Load Sharing

Management Design Guide With MCP73871

AN1260

DS01260B-page 2  2009-2011 Microchip Technology Inc.

MCP73871 DEVICE DESCRIPTION

The MCP73871 device is a fully integrated linear

solution for system load sharing and Li-Ion/Li-Polymer

battery charge management, with AC-DC wall adapter

and USB port power sources selection. It is also

capable of autonomous power source selection

between input or battery. Along with its small physical

size, the low number of required external components

makes the device ideally suited for portable

applications. The MCP73871 device automatically

obtains power for the system load from a single-cell

Li-Ion battery or an input power source (AC-DC wall

adapter or USB port). The MCP73871 device

specifically adheres to the current draw limits governed

by the USB specification. With an AC-DC wall adapter

providing power to the system, an external resistor sets

the magnitude of 1A maximum charge current, while

supporting up to 1.8A total current for the system load

and battery charge current.

The MCP73871 device employs a Constant Current/

Constant Voltage (CC/CV) charge algorithm with a

selectable charge termination point. The constant

voltage regulation is fixed with four available options:

4.10V, 4.20V, 4.35V, or 4.40V to accommodate the

new, emerging battery charging requirements. The

MCP73871 device also limits the charge current based

on die temperature during high-power or high-ambient

conditions. This thermal regulation optimizes the

charge cycle time, while maintaining the device

reliability.

The MCP73871 device includes a low-battery indicator,

a power-good indicator and two charge status

indicators that allows for outputs with LEDs or

communication with host microcontrollers. The

MCP73871 device is fully specified over the ambient

temperature range of -40°C to +85°C.

This application note shows how to design a simple

system load sharing and battery management system

with Microchip’s popular MCP73871 for cost-sensitive

applications.

For more in-depth documentation on these subjects

please refer to Section “References”.

EXAMPLE OF BATTERY CHARGER

AND SYSTEM LOAD SHARING

DESIGN SPECIFICATIONS

The example system that will be applied in this

application note requires an average of 100 mA load

current, and consumes a maximum of 500 mA peak

current for a short duration of time. A 950 mAh rated

Li-Ion battery is used to operate the example system.

The system continuously operates while charging the

Li-Ion battery. The input power supply supplies the

system load and charges the battery when a battery is

present in the system. When the input power source is

removed, the system is supported by the battery. When

the system load and the battery charge current requires

more energy than the supply current can afford, the

system load has higher priority than the battery

charger.

LI-ION/LI-POLYMER BATTERIES

Important attributes when selecting a battery are:

• Internal Resistance

• Operational Load Current

• Energy Density (Size and Weight)

• Charge/Discharge Cycles (Life Cycle)

• Capacity (dominates the operational duration

without external power source present)

As with most engineering work, these key attributes do

not coincide with a reasonable cost. There is always a

trade-off between them when selecting the battery

chemistry for a portable application. Please refer to

Microchip’s AN1088 – “Selecting the Right Battery Sys-

tem for Cost-Sensitive Portable Applications While

Mainta in ing E x cell e nt Qu al i ty” for the details of battery

chemistry comparisons. [5]

Note: The above information is available in the

MCP73871 data sheet (DS22090).

Note: Customers should contact their distribu-

tor, representative or field application

engineer (FAE) for support. Local sales

offices are also available to help custom-

ers. A listing of sales offices and locations

is included at the back of this document.

Technical support is available through the

web site at: http://www.microchip.com/

support. The information in this applica-

tion note is for reference only. Product

design and production are the customer’s

responsibilities.